Phospholemman (PLM) is a 72 amino acid membrane phosphoprotein that interacts with other proteins to participate in transmembrane flux of ions and osmolytes. It is a major substrate for phosphorylation by cAMP-dependent protein kinase (PKA) and protein kinase C in heart and skeletal muscle and may be a central regulatory element in cell signaling through protein kinases. Very little, however, is known about how phosphorylation regulates PLM. The long-term objectives of the proposed work are to understand the effect of phosphorylation on PLM. Our preliminary studies show that PLM interacts with other cell signaling molecules, and that phosphorylation by PKA increases PLM quantity and activity in oocytes and in transfected mammalian cells. To learn more about regulation of PLM by phosphorylation, we propose two specific aims. The first aim is to determine the role of phosphorylation in modulating protein-protein interactions between PLM and dimerization partners such as MLP, a muscle lim protein, and a novel kinase anchoring protein from oocyte that have been identified in yeast two-hybrid assays. To complete this aim, we will study co-precipitation of PLM and partner proteins in transfected mammalian cells and in RNA-injected Xenopus oocytes, and measure ion currents and volume regulation of the cells. The hypothesis is that phosphorylation of different sites in PLM will alter the protein-protein interactions. The second aim is to determine the mechanism by which activation of PKA increases PLM quantity and activity in RNA-injected frog oocytes and in transfected mammalian cells. To complete this aim, we will study transfected cells expressing wild-type and mutant PLMs using biochemical, electrophysiological and volumetric means. The hypothesis is that phosphorylation at serine 68 of PLM leads to increased protein expression and/or stability. Successful completion of these aims will contribute greatly to our understanding of how phosphorylation regulates cell function through PLM, particularly in heart and skeletal muscle where the functional impact of the adrenergic system is profound. In addition, new knowledge on cell signaling through protein kinases will be generated.